Drag line bucket control

ABSTRACT

A dragline excavation bucket control system including a pair of hoist ropes and a drag rope, the hoist ropes being coupled adjacent opposite ends of the bucket. The hoist ropes are supported on spaced sheaves on an excavation boom whereby the hoist ropes extending between the boom and bucket are substantially parallel and the boom support points and bucket attachment points for the hoist ropes are controllable to maintain an optimal carry attitude for a bucket when in use.

THIS INVENTION is concerned with improvements in bucket control systemsfor dragline excavators.

The invention is particularly, although not exclusively, concerned withbucket dump control systems for dragline excavators.

A typical dragline bucket is controlled by two cables or ‘ropes’—a hoistrope, and a drag rope.

It is noted that where a singular ‘rope’ is referred to herein, thismay, and often does, refer to two or more equalised ropes travellinguniformly and performing identical functions.

The hoist rope is pivotally connected via a load equalizer and hoistchains to trunnions towards and on opposite sides of the rear of thebucket and extends over a sheave at the tip of the excavator boom to thedrum of a winch.

The drag rope is coupled via a drag linkage to draw chains in turncoupled on opposite sides of the open mouth of the bucket. Also coupledto the drag linkage is a dump control cable which extends over a dumpsheave attached to the hoist load equalizer and back to a mounting lugon a transverse arch extending over the open mouth of the bucket or tothe sides of the bucket front. The drag rope extends unsupported betweenthe drag drum of the winch and the drag linkage coupled by draw chainsto the front of the bucket.

It is widely held that dragline buckets possess three degrees offreedom—the x and y axes, and the carry angle of the bucket.

In a conventional two rope dragline, the vertical and horizontalpositions of the bucket are controlled by the paid out length of thehoist rope and the drag rope. The bucket carry angle is controlledimplicitly by the relative lengths of the draw chains, hoist chains,dump rope and connecting links, and the positional masses of the bucket,rigging and payload.

Due to the geometric balance, the carry angle reduces as the bucketmoves from the base of the boom to vertically under the boom point. Themaximum payload carried by the bucket occurs for only a narrow band ofcarry angle, with reduced payloads for carry angles higher and lowerthan this band. Accordingly, the carry angle is at best a compromisebetween the bucket geometry rigging design and operational requirements.

The dump zone for the bucket is determined by trigonometric stability ofthe loaded bucket. Generally speaking, at a predetermined distance alongthe boom, usually more than two thirds of its length, the tensions inthe drag rope, draw chain and dump rope, reduce to the point where thedump rope force is no longer sufficient to support the front of thebucket, which rotates about the hoist trunnions to dump the bucket load.

The compromise in bucket carry angle means that efficiencies in theexcavation process are lost by bucket spillages, particularly when thebucket is hoisted either close to the base of the boom or more thanhalfway along the boom. Another limitation of such a rigging design isthat generally it is not possible to dump either inside or outside theimplicit dump radius controlled by the geometric balance mentionedafter.

A prior art two rope—bucket rigging system is described generally inAustralian Patent Application No 28097/99 which relates to an improvedbucket rigging for a conventional two rope system.

Australian Patent Application No 34502/89 proposes a three cable bucketcontrol system having two hoist ropes and a drag rope. In this proposal,the effective paid out length of the two hoist ropes are independentlycontrollable. This system suggests three controllable degrees of freedomand avoids the compromises with the bucket carry angle of the two ropesystems.

The hoist ropes extend over respective sheaves at the tip of the boom,one such hoist rope being coupled via hoist chains to the hoisttrunnions of the bucket. The other hoist rope is coupled to the mountinglug on the transverse arch over the mouth of the bucket.

The bucket is moved from a loaded transport position to a dump positionby shortening either of the rear mounted or front mounted hoist ropesrelative to the other to achieve load dumping from the open mouth of thebucket or rearwardly through the selectively operable hatch. Independentcontrol of the paired hoist ropes is achieved by a radial arm pivoted onthe boom support tower. The radial arm has a sheave mounted on the freeend over which one of the hoist ropes passes. A hydraulic cylinder isactuable to move the radial arm and sheave whereby one hoist rope isshortened relative to the other.

When the bucket is in a horizontal attitude, the bucket support isrepresented by a triangulated support surface having one support pointat the tip of the boom, another support point at the hoist trunnions,and the third support point at the mounting lug on the bucket arch.

The three rope system is potentially superior to the two rope system inthat its effective excavation radius is greater and it permits a greaterdegree of selectivity in the dump zone position. Also, the spillageresulting from carry angle variations during carrying can be reduced byreducing the angle variation.

Again, while generally effective for its intended purpose, theabovementioned apparatus nevertheless suffers a number of shortcomings.

In particular, in order to dump a loaded bucket, a substantial amount ofenergy is required to elevate either the front of the loaded bucketrelative to the rear or vice versa.

The main problem, however, in a three rope system is that whiletheoretically providing a greater degree of control over the bucketcarry angle over a greater boom slew radius, implementation of a controlsystem to manage the relative rope tensions is considered to be anextremely difficult task.

Accordingly, it is an aim of the present invention to overcome orameliorate at least some of the shortcomings or disadvantages of priorart dragline excavator control systems.

According to one aspect of the invention there is provided a draglineexcavator bucket control system, said system comprising:

-   -   a pair of hoist ropes and a drag rope, said system characterized        in that said hoist ropes are supported on said boom adjacent a        free end thereof at spaced support positions and said hoist        ropes are coupled adjacent opposite ends of a dragline bucket        whereby said hoist ropes are substantially parallel and the line        connecting said boom support points and the line connecting said        bucket attachment points are substantially parallel when said        bucket is in an optimal carry attitude for said bucket.

Suitably said control system comprises a support system having fourspaced support points in side elevation forming a quadrilateral shape.

Preferably, in use, said four points of said support system define asubstantially parallelogram shape.

Preferably said bucket, in use, is urged between a transport positionand a dumping position by a dumping means, said dumping means beingoperable by lengthening one of said hoist ropes relative to the otherhoist rope whereby gravitational forces cause movement of said bucketbetween a transport position and a dumping position.

If required, lengthening of one hoist rope relative to the other hoistrope may be effected by separately controllable hoist rope drums.

The separately controllable hoist rope drums may be operated by a commondrive.

If required the separately controllable hoist rope drums may be operatedby respective drives.

Suitably the separately controllable hoist rope drums may be coupled bya selective engagement mechanism to permit, in use, a predetermineddegree of differential relative rotation between said separatelycontrollable hoist rope drums.

The selective engagement mechanism may comprise a clutch mechanism.

Alteratively the selective engagement mechanism may comprise adifferential gear assembly.

Alternatively, the bucket, in use, is urged between a transport positionand a dumping position by relative movement between spaced upper supportpositions for said hoist ropes.

If required, a self compensating hoist rope take up system restores thebucket to a carry position under the influence of potential energystored in said hoist rope take up system.

The self compensating hoist rope take up system may comprise a suspendedmass.

If required, the take up system may comprise a spring biassing means.

Alternatively, the take up system may comprise a hydraulic biassingmeans.

Alternatively the bucket, in use, may be urged between a transportposition and a dumping position by a powered system effective to causerelative shortening of one hoist rope relative to the other.

If required, one of said hoist ropes may be shortened relative to theother by a sheave mechanism contactable with said hoist rope.

Suitably, one of said hoist ropes may be shortened relative to the otherby selective rotation of a sheave support arm pivotally mounted adjacenta free end of an excavator boom.

In order that the invention may be more readily understood and put intopractical effect, reference is now made to a preferred embodimentdescribed in the accompanying drawings in which:

FIG. 1 shows schematically in side elevation a conventional two ropebucket rigging system;

FIG. 2 shows schematically a prior art three rope ‘triangulated’ riggingproposal;

FIG. 3 shows schematically in side elevation a parallel rigging systemaccording to the invention;

FIG. 4 shows one embodiment of a boom end adapted to support a pair ofhoist ropes in a parallel configuration;

FIG. 5 shows an alternative embodiment of the arrangement of FIG. 4;

FIG. 6 shows schematically a side elevational representation of aparallel bucket rigging;

FIG. 7 shows schematically the maintenance of bucket attitude as thedrag rope is tensinoed to move the bucket;

FIG. 8 shows schematically one form of self compensating take up systemfor rigging the bucket after dumping;

FIG. 9 shows an alternative to the embodiment of FIG. 8;

FIG. 10 shows yet another alternative to the device of FIG. 8 or FIG. 9;

FIG. 11 shows schematically a means of dumping a bucket by relativemovement between upper supports of respective hoist ropes;

FIG. 12 shows schematically an alternative means of dumping a bucket bychanging relative hoist rope lengths;

FIGS. 13, 13a show a powered hoist rope shortening mechanism;

FIGS. 14, 14a show an alternative powered hoist rope shorteningmechanism.

FIG. 15 shows yet another mechanism for effecting relative shortening ofone hoist rope to the other.

FIG. 16 shows schematically one form of separately controllable hoistrope drums.

FIG. 17 shows an alternative embodiment to that of FIG. 16.

FIG. 1 shows schematically a conventional bucket excavator riggingwherein excavator 1 comprises a support mast 2, a boom 3 and a bucket 4supported on a hoist rope 5 in turn connected to a hoist rope winch (notshown).

Hoist rope 5 terminates in a coupling (not shown) which connects hoistchains 6 to trunnions 7 towards the rear end of bucket 4. The couplingalso connects a dump sheave 8 over which passes a dump control rope 9connecting at one end to the arch 10 of bucket 4 and at its other end toa drag coupling (not shown) which couples the free end of a drag rope 11to drag chains 12 connected to respective mounts (not shown) on bucket4.

In use, the bucket carry angle is a function of the geometry of thevarious coupling points and respective tensions in the hoist rope, hoistchains, drag rope, drag chains and the control rope.

FIG. 2 shows schematically a three rope system of the type proposed inAustralian Patent Application No 34502/89. In the drawings likereference numerals have been employed for like features.

As can be seen, the use of an additional hoist rope 5 may permitsubstantial savings in rigging mass by dispensing with the heavy hoistcoupling (or equalizer), dump sheave, dump chains and dump control ropeetc.

FIG. 3 shows schematically a side elevational view of a three ropesystem according to one aspect of the invention. Again, like referencenumerals have been employed for like features.

In the embodiment shown a pair of hoist ropes 5, 5a are paid offopposite ends of a hoist winch (not shown) and respectively pass over a‘normal’ boom sheave 20 and an ‘extended’ boom sheave 21 and a secondboom sheave 22 mounted coaxailly with sheave 20.

‘Extended’ boom sheave 21 is mounted on a jib spacer frame 23 to spacehoist ropes 5, 5a in a parallel manner as shown.

By suspending the bucket from front and rear trunnions by parallel hoistropes of effectively substantially equal length, it will be apparentthat the bucket carry attitude will not be influenced to a great extentby drag rope tension and thus independent control of hoist ropes 5, 5afor maintaining bucket attitude is alleviated.

FIG. 4 shows schematically an enlarged view of the end of the boomillustrated in FIG. 3. The jib spacer frame 23 is rigidly mounted onboom 3.

FIG. 5 shows an alternative embodiment to the arrangement of FIG. 4wherein the jib spacer frame 23 is pivotally mounted at its inner end23a to boom 3.

The angular position of frame 23, and thus the relative spacing betweenhoist ropes 5, 5a, may be adjustable by a tensionable cable 24 whichextends over a spacer arm 25 attached to frame 23 and pivotabletherewith. By adjusting the relative spacing between hoist ropes 5, 5a aparallel rope support can be provided for the bucket over a substantialextend of the boom to maximize bucket carrying capacity and to extendboth excavation and dump radii.

If required the fixed jib spacer frame 23 of FIG. 4 may betelescopically adjustable to vary the spacing between hoist ropes 5, 5aas required. Alternatively the pivotable jib spacer frame 23 of FIG. 5may be telescopically adjustable.

FIG. 6 shows in a schematic sense the parallelogram shape defined by thefour support points for the bucket.

Point A represents sheave 22, point B represents sheave 21 as shown inFIGS. 3, 4 and 5, while points C and D represent respectively front andrear bucket trunnions.

FIG. 7 shows that as the drag rope 11 is tensioned to carry the bucketinwardly and upwardly to the position shown in phantom, the angle of thefront and rear bucket trunnions, represented by the line extendingbetween points C and D, remains substantially constant.

FIG. 8 shows a suspended mass 30 coupled, say, to hoist rope 5a via apair of fixed sheaves 31 attached to the excavator (not shown) and afloating sheave 32 to which the mass 30 is attached.

With floating sheave 32 in an extended position as shown to take upslack in rope 5a, a sheave brake (not shown) or other suitable brakingmechanism associated with fixed sheave 31 is engaged to retain the fixedand floating sheaves 31, 32 in their relative positions in turn tomaintain the bucket carry attitude as shown generally in FIGS. 6 and 8.

When the bucket is full and positioned over a desired dump zone, thesheave brake associated with sheave 31 is disengaged to allow rope 5a tobe paid out.

As rope 5 is stationary and maintains a fixed tension on the winch drum,the gravitational force of the loaded bucket forward of the rear hoisttrunnions is such as to cause the bucket to tilt about the rear hoisttrunnions as the tension in the rope 5a overcomes the restoring force ofmass 30. The buckets rotates about its rear trunnions to an uprightposition to dump its load and when the bucket is empty, the mass 30 issufficient to apply a restoring force against the forward portion of thebucket to take up stack in rope 5a to return the bucket to a normalcarry position to continue the excavation process. Once the bucket hasreturned to the normal carry attitude, the sheave brake, or the like, isagain engaged to lock the take up system.

FIG. 9 shows an alternative embodiment of the system of FIG. 8. In thisembodiment, the mass 30 is reduced and is combined with a springmechanism 33 which, when compressed, provides a restoring force toreturn the bucket to its normal carry attitude. The spring mechanismmay, for example, comprise a compression/tension spring of fixed orvariable rate and include a damper during pay out or take up of slackduring the bucket dump and restoration steps.

FIG. 10 shows yet another embodiment incorporating a mass 30, ahydraulic piston/cylinder assembly 34 and a pressure accumulator 35.

Like the apparatus of FIGS. 8 and 9, the restoring forces of mass 30 andthe pressurized accumulator 35 are sufficient to return an empty bucketto its normal carry attitude but are insufficient to resist the tensileload applied to rope 5a when the bucket is full. The hydraulic mechanismof FIG. 10 can be adapted to provide finely tuned dumping in both thecable slack pay out and take up modes. The hydraulic mechanism can alsobe used to provide the sheave locking functions.

FIGS. 11 and 12 show schematically the alternative bucket dumping modesaccording to he invention.

In FIG. 11 the parallelogram shape represented by points A B C D willmove to the parallelogram shape represented by points A C E F when theupper support points A and B are rotated relative to each other. Forexample, this dumping mode may be effected by the embodiment of FIG. 5where the take up mechanism is coupled to control cable 24 to movesupport point B in the parallelogram shape.

While FIG. 11 shows pivoting of support points about point A, thepivoting could be about any point between points A and B, or near them.Some pivot points, in particular, will allow dumping and return to thedesired carry angle through the balance of forces on the full and emptybuckets and without extra power application required.

FIG. 12 shows the change from carry attitude parallelogram points A B CD to dump quadrilateral points A B G H when the relative lengths ofsupport ropes 5, 5a change. In this embodiment, any of the take up unitsof FIGS. 8, 9 or 10 could be employed to cause hoist rope 5a to lengthento enable the bucket to dump its load.

FIGS. 13, 13a and 14, 14a show alternative dumping mechanisms in aschematic sense.

In FIG. 13 one of the hoist ropes 40, either the front or rear, may bepassed between a pair of sheaves 41, 42 mounted on a rotatable frame(not shown) attached to the boom of the excavator. It will be noted thatto reduce rope wear, sheaves 41, 42 are not normally in contact withhoist rope 40.

When it is required to dump the excavator bucket the hoist rope 40 isshortened relative to the other hoist type (not shown) by rotating theframe, to which sheaves 41, 42 are attached, through about up to 180°whereby the sheaves contact the hoist rope and impart a pair of loopstherein to shorten that rope relative to the other hoist rope to effecteither front or rear dumping from the bucket.

FIGS. 14, 14a show an alternative rope shortening mechanism wherein rope45 normally passes between sheaves 46, 47, 48 without contact.

When it is desired to dump the bucket by shortening hoist rope 45relative to the other rope (not shown), sheave 46 is urged betweensheaves 47 and 48 by a suitable mechanical or fluid powered means toform a shortening loop in hoist rope 45.

FIG. 15 shows schematically an alternative embodiment to that shown inFIG. 5.

Hoist ropes 50, 50a pass over respective sheaves 51, 51a mounted atopposite ends of a jib spacer frame 52 which is pivoted intermediate itsends to boom 53 about the pivotal axis of “normal” sheave 54 or at leaston a pivot pin occupying the pivotal axis 55 previously occupied bysheave 54.

A pivot bearing (not shown) associated with jib spacer frame 52 may beslidably mounted in jib spacer frame 52 to selectively position thepivotal axis of frame 52 closer to one of sheaves 51, 51a as required.

If required either or both of the portions of jib spacer frame 52 lyingon opposite sides of pivotal axis 55 are telescopically adjustable bymechanical and/or hydraulic mechanisms.

FIG. 16 shows schematically a cross sectional elevation of a hoist ropecontrol system having separately controllable hoist rope drums.

The drive system 60 includes hoist rope drums 61, 62 rotatable onrespective drive shafts 63, 64. Ring gears 65, 65a are secured intofacing drum wall flanges 61a, 62a which ring gears 65, 65a are coupledto planetary gears 66, 66a in turn coupled to drive gears 67, 67a keyedor otherwise secured on respective drive shafts 63, 64. Planetary gears66, 66a in turn coupled to drive gears 67, 67a keyed or otherwisesecured on respective drive shafts 63, 64. Planetary gears 66, 66a aresecured in a planet cage 68 for rotation about an axis 69 in which driveshafts 63, 64 lie. Plant cage 68 suitably includes gear teeth extendingabout its outer periphery for engagement by a drive train (not shown)coupled to a drive motor or the like (not shown).

Rotation of planet cage 68 causes rotation of drums 61, 62 by adifferential action whereby when shafts 63, 64 rotate or are constrainedto rotate at the same speed, the rotational speed of drums 61, 62 willbe the same. By controlling shafts 63, 64 to operate at differingrelative rates of rotation, drums 61, 62 will selectively rotate atdifferent speeds. Selective control of hoist rope drum rotational speedstherefore can be employed to selectively change the relative lengths ofthe front and rear hoist ropes to urge the bucket from a transportattitude to a dumping position as required.

For example shaft 64 may be secured against rotation by a selectiveengagement mechanism such as a lockable dog clutch or keyed coupling 70secured to the dragline structure 71. Shaft 63 is coupled to a selectiveengagement mechanism 72 such as a friction clutch, powered worm wheelgear train or any suitable mechanism to permit selective locking of drum61 on selective rotation in either direction of drum 61 relative to drum62.

FIG. 17 shows schematically an arrangement for independent driving ofhoist rope drums.

The hoist rope control system comprises separate hoist rope drums 80, 81coupled to respective drive motors 82, 83 by respective gearboxes orpower transmission mechanisms 84, 85.

Selective relative rotation between drums 80, 81 may be effected bycontrol of drive motors 82, 83 and/or by selective control of powertransmission mechanisms 84, 85.

Hoist rope control may be effected by a computer 86 coupled to drivemotors 82, 83 and/or power transmission mechanisms 84, 85 to coordinatehoist rope control for translational movement of a loaded bucket at anoptimum transport or carry angle for that particular bucket and also tocontrol dumping of the bucket at a predetermined position withprecision. A plurality of sensors 87, 88, 89 may also be coupled tocomputer 86 to provide information relating to such characteristics asboom slew angle, boom elevation angle, hoist rope tensions, status ofhoist rope length control mechanisms, actual bucket travel or carryangle, boom slew velocity, hoist rope cable speed or the like.

From the foregoing description it will be apparent that the ‘parallel’rigging arrangement in combination with the cable take up unit providessubstantial improvements over prior art dragline bucket rigging systems.These improvements include increased bucket payload through reducedrigging mass, increased efficiency through reduced spillage from thebucket, greater excavator range and greater dump zone range.

Possibly the most significant advantage is that with relativelyinexpensive adaptations to a conventional dragline excavator, all of theabove improvements may be achieved along with a more energy efficientbucket dumping method which relies on the potential energy in a loadedbucket to dump the load and stored potential energy in a rope take upsystem to restore the bucket automatically to the correct carryattitude.

A rear dumping bucket is preferred as it is readily dumped at anyposition between adjacent the fairleads of the excavator and the boomtip. At the boom tip, a rear dumping bucket can increase the effectivedumping radius by about 3-4 meters compared with a front dumping bucket.

Generally speaking while the apparatus descried herein can be adapted todump either from the front or the rear of a bucket, front dumping isgenerally only effective for the outer half of the excavator boom.

By employing a rear dumping mode of operation by shortening the fronthoist rope relative to the rear hoist rope, excessive tensions in therear hoist rope are avoided and generally rope life can be extended.

It readily will be apparent to a person skilled in the art that manymodifications and variations may be made to the various embodimentsdescribed herein without departing from the spirit and scope of theinvention.

For example, the excavator may include a single hoist rope winch with asingle drive for a pair of hoist ropes. Alternatively, the winch mayinclude multiple drums with independent drives or combinations thereof.In such an example, the winch drums may be operated in unison for thedig and carry operations and separately to control dumping functionsand/or carry angle of the bucket.

Although a number of alternative mechanisms are described herein foreffecting relative lengthening or shortening between the spaced hoistropes, it also will be apparent to a person skilled in the art thatvarious combinations of relative rope length changing mechanisms may beemployed to control bucket carry angle and/or bucket dumping functions.

1. A dragline excavator bucket control system, said system comprising: apair of hoist ropes and a drag rope, respective free ends of said hoistropes being coupled adjacent opposite ends of a dragline bucket and saiddrag rope being coupled adjacent a front end of said dragline bucket,said hoist ropes being supported on an excavator boom on spacedrespective inner and outer boom sheaves; and a boom sheave support armmounted adjacent a free end of said excavator boom to support said outerboom sheave to one side of a longitudinal axis of said boom whereby, inuse, said pair of hoist ropes extending between respective sheaves andrespective couplings to said bucket remain substantially parallel toretain said bucket in an optimal transport attitude when moving betweena retracted and an extended position under the influence of said dragrope.
 2. A control system as claimed in claim 1 wherein in use,respective hoist rope boom support points and respective hoist ropebucket attachment points together define a substantially parallelogramshape in side elevation.
 3. A control system as claimed in claim 1wherein said boom sheave support arm is rigidly mounted on said boom. 4.A control system as claimed in claim 1 wherein said boom sheave supportarm is pivotally mounted on said boom.
 5. A control system as claimed inclaim 1 wherein said bucket, in use, is urged between a transportposition and a dumping position by a dumping mechanism, said dumpingmechanism being operable by lengthening one of said hoist ropes relativeto the other hoist rope whereby gravitational forces cause movement ofsaid bucket between a transport position and a dumping position.
 6. Acontrol system as claimed in claim 5 wherein lengthening of one hoistrope relative to the other hoist rope is effected by separatelycontrollable hoist rope drums.
 7. A control system as claimed in claim 6wherein the separately controllable hoist rope drums are operated by acommon drive.
 8. A control system as claimed in claim 6 wherein theseparately controllable hoist rope drums are operated by respectivedrives.
 9. A control system as claimed in claim 6 wherein the separatelycontrollable hoist rope drums are coupled by a selective engagementmechanism to permit, in use, a predetermined degree of differentialrelative rotation between said separately controllable hoist rope drums.10. A control system as claimed in claim 9 wherein the selectiveengagement mechanism comprises a clutch mechanism.
 11. A control systemas claimed in claim 9 wherein the selective engagement mechanismcomprises a differential gear assembly.
 12. A control system as claimedin claim 4 wherein the bucket, in use, is urged between a transportposition and a dumping position by pivotal movement of said boom sheavesupport arm to effect a lengthening of one of said hoist ropes relativeto the other hoist rope whereby gravitational forces cause movement ofsaid bucket between a transport position and a dumping position.
 13. Acontrol system as claimed in claim 5 wherein a self-compensating hoistrope take up system restores the bucket to a carry position under theinfluence of potential energy stored in said hoist rope take up system.14. A control system as claimed in claim 13 wherein theself-compensating hoist rope take up system comprises a suspended mass.15. A control system as claimed in claim 13 wherein the take up systemcomprises a spring biassing mechanism.
 16. A control system as claimedin claim 13 wherein the take up system comprises a hydraulic biassingmechanism.
 17. A control system as claimed in claim 13 wherein saidhydraulic biassing system includes a pressure accumulating chamber. 18.A control system as claimed in claim 13 wherein the self-compensatingtake up system is selected from a suspended mass, a spring biassingmechanism, a hydraulic biassing mechanism, or combinations thereof. 19.A control system as claimed in claim 5 wherein the bucket, in use, isurged between a transport position and a dumping position by a poweredsystem effective to cause relative shortening of one hoist rope relativeto the other.
 20. A control system as claimed in claim 19 wherein one ofsaid hoist ropes is shortened relative to the other by a respectivepowered hoist rope drum.
 21. A method of operating a dragline excavatorwherein a pair of hoist ropes are coupled adjacent opposite ends of adragline bucket, said hoist ropes being supported on an excavator boomon spaced respective inner and outer boom sheaves, said outer boomsheave being supported by a boom sheave support arm to one side of alongitudinal axis of said excavator boom whereby, in use, said pair ofhoist ropes extending between respective sheaves and respectivecouplings to said bucket remain substantially parallel to retain saidbucket in an optimal transport attitude when moving between a retractedand an extended position under the influence of a drag rope coupled tosaid bucket.
 22. A method as claimed in claim 21 wherein said bucket isurged between a transport position and a dumping position by selectivelylengthening or shortening of one of said pair of hoist ropes relative tothe other hoist rope of said pair.
 23. A method as claimed in claim 21wherein each of said pair of hoist ropes is coupled to a respectiveseparately controllable hoist rope drum.
 24. A method as claimed inclaim 23 wherein each hoist rope drum is selectively operable from acommon drive.
 25. A method as claimed in claim 23 wherein each hoistrope drum is selectively operable by a respective drive.